I associate this sort of thing, from long ago pre-internet days, with Ian Catford, who was able to do this for new phrases more or less on demand. My experience with his displays of this ability go back so far that the preferred way to demonstrate it was to play a reel-to-reel tape backwards.

And I'm not the only one with this memory of him: looking for a link to Ian on the web, I found this:

Ian is also famous for his amazing ability to repeat speech backwards. When asked about this, he reported that he started studying phonetics at the age of 14, and his phonetic skills helped him to gain a position with the BBC as a radio actor, starting when he was 17 – often speaking in multiple exotic dialects. Also, in a production of Alice Through the Looking Glass, he recorded ""Jabberwocky"" both forwards and in reverse. The BBC reversed his reverse performance and it came out quite intelligibly with an appropriate other-worldly quality.

Alas, the link is to Ian's obituary: he died on Oct. 6, a bit more than three weeks ago.

To this untrained ear, the result sounds in places like someone pretending to speak Russian. Is that fair comment? What I like about this is that it lacks the wooop and waarp effects that comes with playing tapes backwards: does anyone know what cause the latter? They are apparent even in the reversed version of this fellow's backward singing.

Graeme, I'm just guessing here but I suspect the distinctive sounds of audio being played backwards come from sounds that forward consist of a sharp attack and then decay. For instance, imagine the sound of a cymbal crash, it starts all of sudden and quickly fades down to a hiss of white noise and then is no longer audible. Played in reverse you get a slight hiss followed by an abrupt crescendo up to an immediate cutoff. That's an extreme example but there are probably lots of natural sounds with that basic profile.

I hear the Russian "accent" both forward and backwards, with the dark l, and palatized vowels. I thought for a bit that Russian initial sounds that are against English phonology (like кн) were responsible, but now I think he's just making those sounds, in either order.

@Peter Seibel: Yes, the attack/decay profiles of many speech sounds are quite asymmetrical – sudden attack, slow decay – and reversing that is responsible for some of the impression that reverse speech makes. I think that would be true in any language.

@Aviatrix: Specifically with regard to the dark /l/, this is about English phonology, not backwards speech generally. The /l/ phoneme tends in many varieties of English to be dark in syllable codas (e.g. in peel) and fairly clear in syllable onsets (e.g. in leap). So when you want to produce the phoneme sequence /lip/ so that it will sound like peel when played backwards, you have to make the /l/ dark. (You also have to get the aspiration before the /p/, etc. etc., which is what makes this performance so impressive.)